Power loss. 318 Vs 340 manifolds

-
AdamR

AdamR

Big Member
Joined
Dec 31, 1969
Messages
17,342
Reaction score
562
Location
Bethel, CT
I need to decide what Im doing exhaust wise on the Dart because Im just about ready to start it.

I really dont want to run headers on it, It just looks to stock under the hood and Im more concerned with ease of maintenance then anything right now.

Ive been considering using a set of early 318 manifolds seeing I cant really afford a $300 or $400 set of 34o manifolds right now. So my question is how much power will I loose on my mild motor.

Heres the details.

10:1 (actual) compression 360

Comp XE 268 224/230- 477/480 - 110lsa

302 casting heads
1.88, 1.60 valves
Stock sized 318 ports.

stock 71 340 cast iron intake (spread bore)
stock 72 340 800cfm Thermoquad.

The motor pulls to about 5500rpms

Thats it, Nothing fancy, pretty much stock parts.

So what do you think I will loose over the 340 manifolds ?
 
all most forgot, The rest of the exhaust will be 2.5" pipes with Dynomax Super Turbos.
 
One of the Mopar mags did a test on a 300 HP crate 360 starting with 318 manifolds all the way up to TTI headers. I think the spread was like 28 HP. If I remember correctly the hp manifolds were about 11 hp better then 318 and 7 betther then basic 360 logs. Will try to find it and link it here.
 
I know the article that furz4 is referring to which was done by Mopar Muscle. The difference between the 318 and hi-po 340's was not significant (11 or 13hp) which isn't enough for me to make the jump to the 340 manifolds. A guy offered me his 67-69 340 manifolds for $250 and even tho that's a great price, the work required to get the existing 318 manifolds off, the 340's on and ensuring they don't leak is not worth it in my mind. Others might think it is for 11 or 13hp so call me crazy.
 
Use the 318 manifolds.,

http://www.moparmusclemagazine.com/...pp_0307_mopar_crate_engine_exhaust/index.html

For our baseline test, we fitted the 360 with the lowest of exhaust low,a set of stock small-passage 318 iron exhaust manifolds. Bolted to themanifolds was a set of 21/4-inch head pipe extensions 24 inches long. Soequipped, the 360 put out a respect- able 311 hp at 4,600 rpm and 403lb-ft of torque.

We moved up a notch from the early 318 iron manifolds to late 360 ironlog-style manifolds. The 360 manifolds are nothing to look at, but theinternal passages and outlet are significantly larger than those of the318 pieces. Power was up a little, moving to 314 hp and 407.1 lb-ft oftorque.


Our final test of factory iron was a set of rare and expensive early 340high-performance manifolds. With the 340 exhaust, the 360 inched up to 315 hp and 409.1 lb-ft of torque, which represented very little gain onthis engine.


furz4 said:
One of the Mopar mags did a test on a 300 HP crate 360 starting with 318 manifolds all the way up to TTI headers. I think the spread was like 28 HP. If I remember correctly the hp manifolds were about 11 hp better then 318 and 7 betther then basic 360 logs. Will try to find it and link it here.
Click to expand...
 
With my combo I went from a 11.78 @ 118mph (Slicks, 3" exhaust, SuperComp hearers) to 13.2 @ 105mph (340 Manifolds, 2.5" exhaust and E70-14 RedLines)

I think I could get a bit better time but the E70-14s are a ***** to launch on.
 
cavemanmoron said:
Use the 318 manifolds.,

http://www.moparmusclemagazine.com/...pp_0307_mopar_crate_engine_exhaust/index.html

For our baseline test, we fitted the 360 with the lowest of exhaust low,a set of stock small-passage 318 iron exhaust manifolds. Bolted to themanifolds was a set of 21/4-inch head pipe extensions 24 inches long. Soequipped, the 360 put out a respect- able 311 hp at 4,600 rpm and 403lb-ft of torque.

We moved up a notch from the early 318 iron manifolds to late 360 ironlog-style manifolds. The 360 manifolds are nothing to look at, but theinternal passages and outlet are significantly larger than those of the318 pieces. Power was up a little, moving to 314 hp and 407.1 lb-ft oftorque.


Our final test of factory iron was a set of rare and expensive early 340high-performance manifolds. With the 340 exhaust, the 360 inched up to 315 hp and 409.1 lb-ft of torque, which represented very little gain onthis engine.
Click to expand...


Interesting. Despite this the 340 ones have more fame, more pricey too. For the price you pay for 340 manifolds 4hp doesn't seem like a big loss.
 
Make a template of the 340 heads and lay in over the 318 manifolds, do some grinding to not shroud the ports on the 340 heads with the small 318 factory manifolds. Also measure the outlet of the 318 manifolds, if it real small outlet, open that area up a bit also. I bet doing those two things it would make the 318 manifolds that much closer to the factory 340 manifolds.

Mark Lelchook
Performance Welding
Racing Headers
perfweldheaders.com
 
I do not like that manifold comparison.... They are using a hodgepodge of headers and manifolds....

They are comparing manifolds that will fit in an a-body with ones that won't.... Take the 340 manifold vs 318 manifold info with a grain of salt....

It would only be accurate if we had the numbers form 340 manifold vs 318 manifold for the same application.
 
Go with a cheap set of headers Adam. You can get a set from Summit for about 150 bucks. Meanwhile keep looking for a fair price on a set of manifolds.
 
Put the 318 manifolds on and enjoy!

Our cars are always a WIP (work in process)!:toothy10:
 
Longgone said:
Go with a cheap set of headers Adam. You can get a set from Summit for about 150 bucks. Meanwhile keep looking for a fair price on a set of manifolds.
Click to expand...


I dont feel like disassembling the front suspension to put a set of headers in that I dont really want to use in the first place.
 
power loss depends on the cam you use for 1.

I'd say about 15-19hp, the real deal is the 340 manifolds will allow it to rev and still make power in the upper.

See chrysler eng'rs found minimal power loss as long as they kept the head pipes 3 1/2 + inches straight out from the port, and divided the ports as you can see in the pics. these sets are from a 69 dart and the other is from my 70 duster that someone painted w/high temp.

Photo 426.jpg


Photo 416.jpg


Photo 417.jpg
 
This is an interesting thread as I have been debating the same thing.

Not to hijack but are the hp manifolds from say a '68 dart the same from a '71 cuda? Meaning regardless of car will they all bolt up to any small block in any model wheather a,b,e body?

I have a mild 9:1 318 with 340 cam, edelbrock 4-barrel and intake. I have been reading that the cam choice makes a big difference in both manifolds. Would the hp manifolds help more with a 340 cam than a stock 318 cam or any other cam? Being thats the cam the 340 cars came from the factory with.
 
I have a set of Magnum truck manifolds I bought to use on my 73 Dart and there is no way they will clear power steering. Also the outlet on the driver side and somewhat on the pass side is right up next to the firewall. Head pipe would probably be in way of shift linkage also. The bigger the cam the more the HP manifolds would gain the cam used in 300 HP crate motors is pretty small so real world numbers will be bigger then in the article. It's going to depend on if you want max power or not, then IMO headers are the way to go. But if your only planning on 250-300 hp with a mild 318 build I still think manifolds might be the ticket.
 
how do you run 2.25 tube on drivers side with 340 mainifolds. no room.
Thanks.....jerry
 
Steve68 said:
Has anyone tried something like the Headman Hedders Tight Tube Hedders? (http://www.hedman.com/products/?id=3482&iset=4659&rsku=)
It would be interesting to know if they would fit very well and how they would compare to the stock manifolds.
Click to expand...
i have a set of those on the shelf.they fit well but require a mini starter.im not sure you would gain much over 340 manifolds.another down side is the ports are 4bbl head sized.the earlier 318 2bbl heads dont have as much sealing surface around the exhaust ports as the later smog heads.i would be worried about gasket blow out on the pre AIR 2bbl heads.
 
Adam, go the easiest, cheapest, most reliable route and be done with it. 318 manifolds. Less stress, more kruzin' time ! :cool:
 
Longgone said:
Go with a cheap set of headers Adam. You can get a set from Summit for about 150 bucks. Meanwhile keep looking for a fair price on a set of manifolds.
Click to expand...

A cheap set of headers are way more trouble than they are worth. I'll never install a cheap set on anything ever again after installing the Doug Thorley's on my 440 Coronet. They are by far the best made, fitting headers I have ever used or have seen so far. Expensive, but I got a deal on e-bay if you can believe that!
 
You must log in or register to reply here.
-

Back
Top